The present work reports ammonia (NH3) gas sensing properties of Graphene film synthesized by chemical vapor deposition (CVD) in ambient condition. Graphene with a large area is synthesized on Cu foils by chemical vapor deposition under ambient pressure and the graphene film has transferred into Si/SiO2 wafer. Raman mapping indicates monolayer graphene dominates the transferred graphene film. By fabricating ohmic contact to both side of graphene layer, we measured DC resistance changes due to NH3 gas flow. The gas molecules are absorbed on graphene by chemisorption and resistance increases by electron injection. The sensing characteristics of graphene has been examined by changing the inlet gas flow and the sensor shows better performance with large flow, such as fast response time and high sensitivity.